Longer Heat Streaks Across the U.S.

Aug 16, 2017

By Climate Central

We're in the final weeks of meteorological summer, usually a time when heat has worn out its welcome. Fortunately, for many in the Plains, Midwest, and the East, this August is providing a break from the sweltering heat. But over the long term, as the planet continues to warm from the increase in greenhouse gases, extended streaks of heat are getting longer. In our analysis this week, we examine the number of consecutive hot days annually for these cities. While there’s a lot of variation from year to year in the length of these hot spells, the trend suggests more prolonged periods of high heat in the majority of locations.

Longer streaks of 85°F, 90°F, 95°F, 100°F, 105°F and 110°F in these cities

Exposure to prolonged hot spells can raise the risk of heat-related illnesses like heat stroke, especially in urban areas where the impacts are compounded by the urban heat island effect. Late summer activities can further heighten the risk of heat-caused illness. High school and college football, for example, usually begin practices in pads at this time of year. When exercising, the body produces up to 20 times more heat than when at rest, and the risk of heat-related illness rises after 3 percent of the body’s weight is lost through fluids.

Economically, extended hot streaks increase the electricity demand from air conditioning, raising cooling costs and putting added stress on the electrical grid. They also increase the risk of wildfires, such as the ones that devastated northern Montana last month with multi-million dollar impacts on ranching communities and beef production.

NOTE: Although the term "heat wave" is sometimes used to indicate a stretch of abnormally hot weather, we opted not to use that word here since heat waves have a specific definition that varies across the country.

METHODOLOGY: To find heat streaks, we used the highest temperature (at 5°F intervals) that occurred an average of at least 5 consecutive days per year and at least once a year. Stations that did not meet both criteria or were missing data were evaluated at a lower threshold temperature. Let us know if you would like to see the data calculated at a different threshold temperature.

In some parts of the country, August has lost a little of its bite this year. While still early, the first week and a half of August has been much cooler than normal from an area spanning from the Upper Midwest to the central Gulf Coast. The outlook from NOAA’s Climate Prediction Center through the third week of the month is for much of the same.

The number of these very hot days is expected to increase sharply as the world further warms from the buildup of greenhouse gases. What may initially appear to be a small increase in the average temperature has a large impact on these extremes. As an entire distribution of daily high temperatures shifts slightly warmer, those rare days with extreme

While increasing extreme daytime heat is one of the clearest signs of a warming climate, low temperatures have also been increasing. In many cases, nighttime low temperatures are increasing more rapidly than daytime high temperatures. Take the first half of 2017, the second hottest start to the year on record for the U.S. From January-June, the

Average summer temperature have risen a few degrees across the West and Southern Plains, leading to more days above 100°F in Austin, Dallas and El Paso all the way up to Oklahoma City, Salt Lake City, and Boise. It’s worth noting that this trend has been recorded across the entire Northern Hemisphere, as show in this WXshift animation.